Microwave systems are often composed of monolithic microwave integrated circuits (MMICs), other active microwave devices such as GaAs transistors, passive microwave components and other non-microwave components such as logic and control structures.
A monolithic integrated circuit or MMIC is an integrated circuit which is designed to operate at microwave frequencies. MMICs are normally fabricated in GaAs because of the much higher potential operating frequency which GaAs provides as compared to silicon. A typical MMIC may include one or more amplifiers, some passive components and one or more feedback loops which provide feedback from the output of an amplifier or circuit to establish a desired transfer function for that circuit.
The process of designing an interconnection pattern for interconnecting all of the chips and components of an electronic system on a single high density interconnect substrate normally takes somewhere between one week and five weeks. Once that interconnect structure has been defined, assembly of the system on the substrate may begin. First, the chips are mounted on the substrate and the overlay structure is built-up on top of the chips and substrate, one layer at a time. Typically, the entire process can be finished in one day and in the event of a high priority rush, could be completed in four hours. Consequently, this high density interconnect structure not only results in a substantially lighter weight and more compact package for an electronic system, but enables a prototype of the system to be fabricated and tested in a much shorter time than is required with other packaging techniques.
This high density interconnect structure method of fabricating it and tools for fabricating it are represented in U.S. Pat. No. 4,783,695, entitled "Multichip Integrated Circuit Packaging Configuration and Method" by C. W. Eichelberger, et al.; U.S. Pat. No. 4,835,704, entitled "Adaptive, Lithography System to Provide High Density Interconnect", by C. W. Eichelberger, et al.; U.S. Pat. No. 4,714,516, entitled "Method to Produce Via Holes in Polymer Dielectrics for Multiple Electronic Circuit Chip Packaging" by C. W. Eichelberger et al.; U.S. Pat. No. 4,780,177, entitled "Excimer Laser Patterning of a Novel Resist" by R. J. Wojnarowski et al.; U.S. patent application Ser. No. 249,927, filed Sep. 27, 1989, entitled "Method and Apparatus for Removing Components Bonded to a Substrate" by R. J. Wojnarowski et al.; U.S. Pat. No. 4,894,115, entitled "Laser Beam Scanning Method for Forming Via Holes in Polymer Materials" by C. W. Eichelberger, et al.
Many MMICs and other active microwave devices include delicate structures which can easily be damaged or destroyed. These include conductors which are spaced from the surface of the GaAs by an air gap, a structure which is known as an "air bridge". Air bridges are used in these MMICs in order to provide the MMIC with particular desired operational characteristics. These delicate structures severely limit the assembly techniques which can be used to connect these devices into microwave systems. Further, such components are quite sensitive to the placement near their surfaces of conductors or dielectric materials having dielectric constants of more than one, especially in the vicinity of inductors, air bridges and field effect device gate regions.
Hermetic ceramic packages to protect MMICs have been used for some time for long-term protection of MMICs but with considerable tradeoffs in both cost and weight. A low-cost, light-weight alternative can be offered by plastic packages without compromising long-term device reliability. This is possible through the use of polymers for construction of the cavity as well as filler materials to form a barrier to moisture transport in the package cavity. Though these polymers do not affect the electrical performance of low-frequency devices, their relatively high dielectric permittivity and loss tangent can cause substantial changes in the electrical performance of analog MMICs i.e., frequency shift, inductor Q deterioration, mismatches due to impedance changes.
Most microwave control devices i.e., switches and attenuators, are rendered less sensitive to fillers and moisture by avoiding the use of air-bridges and by adding a final coating of SiN 0.5 um thick. Such devices are currently commercially available and have been qualified for commercial RF/microwave systems. Most analog devices, however, not only contain air-bridges, but are also sensitive to the detuning effects of the dielectric of the dielectric coatings.
Many of these devices with air bridges are so sensitive to the presence of overlying dielectric layers that their manufacturers do not even deposit glass passivation layers on the upper surface of those chips because even that thin dielectric layer would adversely affect the operating characteristics of the components. The use of ceramic glass, like SiC, passivating coatings on semiconductor chips is essentially universal in the silicon semiconductor art to prevent environmentally induced deterioration of the devices. Consequentially, in the microwave art disposing any additional dielectric material on an active device or component is looked upon as a sure way to degrade performance and is assiduously avoided.